The rapid development of information industry, particularly the increasing demand from the mobile Internet and the Internet of things (IoT), brings about unprecedented challenges in the future mobile communications technology. According to the ITU-R M. [IMT.BEYOND 2020.TRAFFIC] issued by the International Telecommunication Union (ITU), it can be expected that, by 2020, mobile services traffic will grow nearly 1,000 times as compared with that in 2010 (fourth generation (4G) era), and the number of user device connections will also be over 17 billion, and with a vast number of IoT devices gradually expand into the mobile communication network, the number of connected devices will be even more astonishing. In response to this unprecedented challenge, the communications industry and academia have prepared for 2020s by launching an extensive study of the fifth generation of mobile communications technology (fifth generation (5G)). Currently, in ITU-R M. [IMT.VISION] from ITU, the framework and overall objectives of the future 5G have been discussed, where the demands outlook, application scenarios and various important performance indexes of 5G have been described in detail. In terms of new demands in 5G, the ITU-R M. [IMT.FUTURE TECHNOLOGY TRENDS] from ITU provides information related to the 5G technology trends, which is intended to address prominent issues, such as significant improvement on system throughput, consistency of the user experience, scalability so as to support IoT, delay, energy efficiency, cost, network flexibility, support for new services and flexible spectrum utilization, and the like.
Modulation waveform and multiple access are important foundations for designing wireless communication air-interfaces, and 5G will be no exception. At present, the typical example orthogonal frequency division multiplexing (OFDM) in the multi-carrier modulation (MCM) technological family has been widely used in the broadcasting-type audio and video fields and the communication systems for civilian use, for example, the long term evolution (LTE) systems corresponding to the evolved universal terrestrial radio access (E-UTRA) developed by the 3rd generation partnership project (3GPP), European digital video broadcasting (DVB) and digital audio broadcasting (DAB), very-high-bit-rate digital subscriber loop (VDSL), institute of electrical and electronics engineers (IEEE) 802.11a/g wireless local area network (WLAN), IEEE802.22 wireless regional area network (WRAN) and IEEE802.16 world interoperability for microwave access (WiMAX) and more. The basic idea of the OFDM technology is to divide a broadband channel into multiple parallel narrow-band sub-channels or sub-carriers, so that a high-speed data stream transmitted in a frequency selective fading channel becomes low-speed data streams transmitted in the multiple parallel, independent and flat-fading sub-channels. In this way, the capability of the system against the multipath fading is greatly enhanced, and OFDM can realize simplified multi-carrier modulation and demodulation by use of inverse fast fourier transform (IFFT) and fast fourier transform (FFT), and then, by adding a cyclic prefix (CP), the linear convolution with the channel becomes circular convolution, and as a result, according to the properties of the circular convolution, when the length of the CP is larger than the maximum multipath time delay of the channel, the inter-symbol interference (ISI) free reception can be realized by the simple single-tap frequency-domain equalization, and the processing complexity of the receiver is thus reduced.
Although CP-OFDM-based waveform can well support the service requirements of mobile broadband (MBB) in 4G era, CP-OFDM shows great restrictions or deficiencies in application scenarios of 5G since 5G will face more challenging and various scenarios. Such restrictions or deficiencies are mainly manifested in the following. First, adding a CP to combat ISI will greatly decrease the spectrum efficiency in 5G low-delay transmission scenarios. This is because low-delay transmission will greatly shorten the symbol length of OFDM, while the length of the CP is only limited by the impulse response of the channel, and in this case a ratio of the length of the CP to the symbol length of OFDM will be greatly increased. As a result, such an overhead causes very large spectrum efficiency loss. Low spectrum utilization is unacceptable. Second, the strict time synchronization requirement will cause a large signaling overhead required by closed-loop synchronization maintenance in IoT scenarios of 5G, and due to the strict synchronization mechanism, the structure of the data frames is not flexible so that it is unable to well support different synchronization requirements of various service. Third, the use of rectangular pulse shaping in OFDM will slow down the frequency-domain side-lobe roll-off and thus result in high out-of-band (OOB) emission. Hence, OFDM is quite sensitive to carrier frequency offset (CFO). However, as for 5G, there will be many demands for flexible access and sharing of fragmented spectrum, the high OOB emission of OFDM significantly restricts the flexibility of spectrum access, or in other words, requires a very large frequency-domain guard band, and as a result, the utilization of spectrum is reduced. Those problems are mainly caused by its inherent characteristics. Although the influence of those problems can be reduced by taking some measures, the complexity of system design will be increased, and it is unable to address those issues fundamentally.
To this end, as described in a report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] from the ITU, some new waveform modulation technologies, for example, multi-carrier-based modulation, have been taken into the consideration of 5G. Among others, the filtered-OFDM (F-OFDM) modulation technology becomes one of the research focuses. The F-OFDM technology introduces time-domain filtering based on OFDM. By the design of a time-domain filter, F-OFDM can significantly reduce the OOB emission caused by filtering of the time-domain rectangular window, and also inherit some unique advantages of OFDM, for example, protection against frequency selective fading by adding a CP based on the complex field orthogonality between the sub-carriers, and the like. Good suppression against the OOB emission can well support the fragmented spectrum. Meanwhile, compared with other new waveform modulation technologies, such as filter-bank multi-carrier (FBMC), by the complex field orthogonality between the sub-carriers, F-OFDM can provide better support to fading channels and multi-antenna systems. F-OFDM supports sub-band filtering, that is, the available band can be divided into non-overlapped sub-bands, and the sub-bands can use a different multi-carrier modulation parameter comprising sub-carrier spacing, CP length or more. In order to avoid the interference between sub-bands, several or no sub-carriers can be inserted between different sub-bands as guard bands which are allocated to different services or different users. The filtering based on sub-bands improves the spectrum utilization of the system and the flexibility of use of spectrum.
Due to the above excellent properties, F-OFDM becomes one of new waveform modulation technological candidates of 5G. However, F-OFDM itself has some problems. Specifically, for F-OFDM, time-domain filtering is performed on OFDM symbols added with a CP, and as a result, the filtered OFDM symbols are extended in time-domain. Meanwhile, in order to improve the performance of reducing the OOB emission of F-OFDM, a long time-domain filter is usually used. For example, in the publication [Filtered OFDM: A New Waveform for Future Wireless Systems], the length of the used time-domain filter is half of that of OFDM symbols. Consequently, the ISI is caused between the adjacent symbols, and the system bit error rate (BER) performance is degraded. Although the influence of this problem can be mitigated by designing a time-domain filter that the main energy of the filter is concentrated within a certain range, in some scenarios, for example, when the bandwidth of a sub-band is narrow or when the used symbol modulation order is high, the effect of ISI caused by the extension of symbols in time-domain due to filtering cannot be ignored, or even worse, may result in an error floor.
In conclusion, in order to improve the competitiveness of F-OFDM as technological candidates of 5G, it is necessary to address its deficiencies in addition to development of its advantageous features. For various scenarios in 5G, particularly for the narrow-band service transmission methods or more in IoT scenarios, it is very necessary to address the ISI caused by the extension of symbols due to time-domain filtering to wireless communication systems in F-OFDM.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.